1. Field of the Invention
This invention generally relates to medical instruments and more specifically to an extractor for removing an object from a body, particularly calculi that can form in the biliary and urinary systems.
2. Description of Related Art
Recently developed medical instruments reduce the invasiveness and potential trauma previously associated with various medical procedures. The removal of calculi in the form of kidney stones, gallstones and the like from the body is one area where this effort is meeting with success. Various instruments now permit the removal of kidney stones and gallstones without the need for major surgery.
Some of these instruments incorporate miniaturized grasping forceps. This invention, however, is directed to an alternative set of instruments that utilize mechanical retrieval baskets as described in the following United States Letters Patent:
4,590,938 (1986) Segura et al PA0 4,611,594 (1986) Grayhack et al PA0 4,625,726 (1986) Duthoy PA0 4,807,626 (1989) McGirr The Segura et al patent discloses a medical retrieval device that can be inserted through the working channel of an endoscope for removing stones and the like from the kidneys or the ureter or biliary duct. It includes a retrieval basket of relatively large diameter that is extendable from the distal end of a sheath and collapsible when withdrawn into the sheath. Outwardly bowed spring strips form the retrieval basket. These strips extend generally axially of the sheath and join at respective distal and proximal ends. The retrieval basket has a generally bulbous form which is relatively stiff due to the spring strip construction and facilitates dislodgement and capture of stones.
The Grayhack et al patent discloses another retrieval basket that is useful during the removal and/or destruction of calculi. A smooth outer tubular sheath overlies and contains a stranded wire cable terminating in a protective tip at the working or distal end of the device. When the cable is extended distally, the spring wire strands at the working end of the cable expand to form a retrieval basket. The distal end of this device additionally includes an expandable distal portion for protecting surrounding tissue during withdrawal of the device and calculi.
The Duthoy patent discloses an extraction device that includes a retrieval basket formed from a plurality of wires spaced about and outwardly from an imaginary extension of the center line of a hollow cable. A filiform extends distally from the distal end of the retrieval basket to extend past a stone and to allow the basket to be threaded around and onto the calculi.
The McGirr patent discloses an extractor included a self-closing retrieval basket at the distal end of a catheter with a flexible control line for opening the basket from the proximal end of the catheter. The basket assumes a normal position wherein it is in a compact closed form. Pulling on the control line flexes the strips to open the basket. When the control line is released, the strips relax and surround the calculi or object being removed.
These and other surgical extractors fusing retrieval baskets have certain common characteristics. Each retrieval basket comprises a plurality of strands in the form of individual strips or wires substantially equiangularly spaced about the retrieval basket. In some retrieval baskets the strands are formed along substantially straight lines when the basket is in a compact form; in others, the individual strands extend along a generally helical path. Each instrument includes a plurality of three or more strands. However, the overall size or diameter of an extractor and ancillary equipment, such as an endoscopic device, can impose upper limits on that plurality. For example, an instrument having a sheath outer diameter of 3.0 Fr can incorporate up to 6 wires each having a diameter 0.008 inches in the prior art.
Additional wires could be advantageous because increasing the number of wires increases the number of contacts between the basket and any entrapped calculi. However, the overall size limitation means that additional wires can be incorporated only by decreasing their diameters. As the individual diameters reduce, they become weaker. Moreover the individual wires are generally equiangularly spaced, so the additional wires reduce the angular spacing between adjacent wires. This can complicate the manipulation of an expanded retrieval basket onto calculi. Wires of reduced diameter can also limit any radially acting, dilating force that the wires exert against surrounding tissue when the retrieval basket expands. In some applications the retrieval basket may not fully open. This attribute can reduce the opening between adjacent wires and make it more difficult to entrap calculi. Even prior art extractors with larger single wires can fail to expand fully with the same result.